The transport Layer is the second layer in the TCP/IP model and the fourth layer in the OSI model. It is an end-to-end layer used to deliver messages to a host. It is termed an end-to-end layer because it provides a point-to-point connection rather than hop-to-hop, between the source host and destination host to deliver the services reliably. The unit of data encapsulation in the Transport Layer is a segment.
Services Offered by Transport Layer
The transport layer provides reliable data transfer services such as segmentation, flow control, error detection, and retransmission, end-to-end communication between devices.
1.End-to-end Connection between Hosts
The transport layer is also responsible for creating the end-to-end Connection between hosts for which it mainly uses TCP and UDP. TCP is a secure, connection-orientated protocol that uses a handshake protocol to establish a robust connection between two end hosts. TCP ensures the reliable delivery of messages and is used in various applications. UDP, on the other hand, is a stateless and unreliable protocol that ensures best-effort delivery. It is suitable for applications that have little concern with flow or error control and requires sending the bulk of data like video conferencing. It is often used in multicasting protocols.
End-to-End Connection2.Flow Control
The transport layer provides a flow control mechanism between the adjacent layers of the TCP/IP model. TCP also prevents data loss due to a fast sender and slow receiver by imposing some flow control techniques. It uses the method of sliding window protocol which is accomplished by the receiver by sending a window back to the sender informing the size of data it can receive.
3. Multiplexing and Demultiplexing
Multiplexing(many to one) is when data is acquired from several processes from the sender and merged into one packet along with headers and sent as a single packet. Multiplexing allows the simultaneous use of different processes over a network that is running on a host. The processes are differentiated by their port numbers. Similarly, Demultiplexing(one to many) is required at the receiver side when the message is distributed into different processes. Transport receives the segments of data from the network layer distributes and delivers it to the appropriate process running on the receiver’s machine.
Multiplexing and Demultiplexing4.Connection Establishment
When two devices in a network wants to establish a connection using TCP, it is done through 3-Way Handshake Process.
- The first computer connects to the second computer by sending a SYN packet to a specified port number.
- If the second computer is listening, it will respond with a SYN/ACK.
- When the first computer receives the SYN/ACK, it replies with an ACK packet.
- After this, the two devices can communicate normally.
5.Connection Termination
In a TCP connection, we have two types of termination mechanisms:
- In the Graceful connection release, the connection is open until both parties have closed their sides of the connection.
- In an Abrupt connection release, either one TCP entity is forced to close the connection or one user closes both directions of data transfer.
Read more about TCP Connection Termination.
6. Reliable Data Delivery
The transport layer checks for errors in the messages coming from the application layer by using error detection codes, and computing checksums, it checks whether the received data is not corrupted and uses the ACK and NACK services to inform the sender if the data has arrived or not and checks for the integrity of data.
Limitations of Transport Layer Services
- The Transport Layer does not handle routing or addressing between the source and destination. It rely on the Network Layer for this.
- Transport layer do not have features like encryption and authentication unless protocols like TLS are used.
- Reliable transmission methods like TCP add extra overhead which can slow down communication.
- The Transport Layer depends on the Application Layer for specific needs and the lower layers for handling network failure.
What are the main protocols used in the Transport Layer?
The two most common protocols are
What is the difference between flow control and congestion control?
Flow control prevents the sender from overwhelming the receiver, while congestion control prevents excessive data from causing network congestion.
What is the role of ports in the Transport Layer?
Ports help identify specific applications on a device. It allows multiple services (like web browsing and email) to run simultaneously over the same network connection.
Why is UDP faster than TCP?
UDP does not establish a connection, send acknowledgments, or retransmit lost data, reducing overhead and making it ideal for real-time applications like VoIP and online gaming.
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